To provide a cross member capable of automatically assembling a cross member to a side member, and improving torsion strength of a chassis frame for an automobile with the cross member assembled to the side member, and a chassis frame for an automobile using it.

Both ends of this cross member are inserted in U-sectioned side members 2, 2', and both ends of the cross member to be fixed are formed into an end shape matched with a sectional inner shape (having an arc-shaped inner corner) of the side member having an outside dimension without a clearance from a sectional inside dimension of the side members 2, 2', so that holes of rivets of both members are precisely matched with each other at the time of assembling the both of the members so as to enable automatic assembly and improve assembling accuracy of the chassis frame for reducing costs. In this chassis frame, the cross member 6 is connected with flanges 2a, 2a' and/or a web 2c of the side members 2, 2', so that particularly torsion strength of the chassis frame assembled to both of the members is improved in addition to the above-mentioned effect.